Semiconductor transistors, in particular field-effect controlled switching devices such as a MISFET (Metal Insulator Semiconductor Field Effect Transistor), in the following also referred to as MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and a HEMT (high-electron-mobility Field Effect Transistor) also known as heterostructure FET (HFET) and modulation-doped FET (MODFET) are used in a variety of applications. HEMTs are preferred in many applications due to their favorable power density, on-state resistance, switching frequency, and efficiency benefits over over conventional silicon based transistors.
HEMTs are typically formed from type III-V semiconductor materials, such as GaN, GaAs, InGaN, AIGaN, etc. In a GaN/AlGaN based HEMT, a two-dimensional electron gas (2DEG) arises at the interface between the AIGaN barrier layer and the GaN buffer layer. The 2DEG forms the channel of the device instead of a doped region, which forms the channel in a conventional MOSFET device. Similar principles may be utilized to select buffer and barrier layers that form a two-dimensional hole gas (2DHG) as the channel of the device. A 2DEG or a 2DHG is generally referred to as a two-dimensional carrier gas.
Epitaxial growth techniques are commonly used to form semiconductor substrates that include the type III-V semiconductor material for the formation of HEMT devices therein. According to one technique, a base substrate that includes readily available semiconductor material, such as silicon or silicon carbide, is provided. Several epitaxial layers of type III-V semiconductor material are formed on the base substrate, Intermediary layers, such as AIN layers, may be used to facilitate epitaxial growth and to improve electrical performance of the substrate.
Recently, epitaxial regrowth techniques are gaining favor as a preferred technique for forming type III-V semiconductor substrates. According to this technique, a type III-V semiconductor layer is epitaxially grown and partially removed. Subsequently, a type III-V semiconductor layer is regrown on the original type III-V semiconductor layer. The regrown layer has improved electrical characteristics in comparison to the original layer. This improvement can lead to an HEMT with lower RDSON (on-resistance) with the same pitch.
One challenge with respect to epitaxial regrowth relates to alignment of the device features that are formed in the regrown layer. In many cases, the regrown epitaxial layer may include structured regions that need to be aligned with structured regions in subjacent layers. Examples of these structured regions include emitter junctions and gate junction.
One technique for aligning structured regions in a semiconductor substrate involves utilizing three dimensional alignment features around the periphery of the active device area. These alignment features provide a reference point from which to ensure that different masks are centered with respect to one another. However, this technique is not compatible with epitaxial regrowth techniques because the epitaxially regrown layer covers the alignment features and makes the alignment features difficult or impossible to distinguish by the processing equipment.